1. Field of the Invention
This invention relates generally to gas turbine engine combustors and, more particularly, to noise reduction in the combustors.
2. Description of Related Art
Air pollution concerns worldwide have led to stricter emissions standards. These standards regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO) generated as a result of gas turbine engine operation. In particular, nitrogen oxide is formed within a gas turbine engine as a result of high combustor flame temperatures. Making modifications to a gas turbine engine in an effort to reduce nitrous oxide emissions often has an adverse effect on operating acoustic levels of the associated gas turbine engine.
Destructive or undesirable acoustic pressure oscillations or pressure pulses may be generated in combustors of gas turbine engines as a consequence of normal operating conditions depending on fuel-air stoichiometry, total mass flow, and other operating conditions. The current trend in gas turbine combustor design towards low NOx emissions required to meet federal and local air pollution standards has resulted in the use of lean premixed combustion systems in which fuel and air are mixed homogeneously upstream of the flame reaction region.
The fuel-air ratio or the equivalence ratio at which these combustion systems operate are much “leaner” compared to more conventional combustors in order to maintain low flame temperatures which in turn limits production of unwanted gaseous NOx emissions to acceptable levels. This method often uses water or steam injection for achieving low emissions, but the combustion instability associated with operation with water or steam injection and at low equivalence ratio also tends to create unacceptably high dynamic pressure oscillations in the combustor that can result in hardware damage and other operational problems. Pressure pulses can have adverse effects on an engine, including mechanical and thermal fatigue to combustor hardware. The problem of pressure pulses has been found to be of even greater concern in low emissions combustors since a much higher percentage of air is introduced to the fuel-air mixers in such designs.
Aircraft engine derivative annular combustion systems, such as the LM series of gas turbine engines from the General Electric Company, with their short compact combustor design have been observed to produce complex predominant acoustic pressure oscillation modes in the combustor. As an example, the LMS100 Rich-SAC (Single Annular Combustor) produces combustion dynamics when injecting water for NOx control. These combustion acoustics can be of high enough amplitude to produce HCF cracking in combustor hardware, as well as drive accelerated wear on combustor interface surfaces. The LMS100 high power, intercooled cycle produces significantly lower T3, higher fuel-air ratios and uses a higher water flow than previous marine and industrial rich-SAC engines, all of which exacerbate combustion acoustics. As a result, the LMS100 is the first M&I SAC incorporating combustion dynamics control design features.
Dry-low-emissions (DLE) combustors are more prone to combustion acoustics and typically include design features and/or control logic to reduce the severity of combustion acoustics. These include quarter wave tubes to dampen pressure fluctuations, multiple fuel systems, and supplemental fuel circuits. Multiple fuel systems allow for flame temperature variation within the combustion chamber.
The LM2500 DLE and LM6000 DLE incorporate three rings of premixers that are independently fueled. This allows for the outer, middle, and inner premixers to have different flame temperatures. The radial variation in flame temperature can be as high as several hundred degrees.
Supplemental fuel circuits have been used to inject a relatively small portion of the fuel into the combustor at different locations from the primary injection locations. The supplemental fuel may have a convective time scale, defined as the time it takes for the fuel/air mixture to travel from the point of injection to the flame front, different than that of the primary fuel source. As such, pressure waves in the combustor are unlikely to interact in the same manner with both fuel sources. This out-of-phase fluctuation in heat release serves to reduce the amplitude of the pressure fluctuations. In some implementations, the supplemental fuel also introduces temperature variation within the combustion chamber.
In the General Electric LM2500 DLE and LM6000 DLE combustors, supplemental fuel is injected from every other premixer. The fuel flow to premixers without supplemental fuel is generally lower than those with the supplemental fuel. The premixer-to-premixer flame temperature variation can be as high as several hundred degrees. It should be noted that a circumferential pattern of supplemental fuel in the LM2500 DLE and LM6000 DLE premixers is constrained by premixer design to every-other premixer.
It is highly desirable to have an effective means for eliminating or reducing these high levels of noise or acoustics in a gas turbine engine combustor, particularly, one that has a short length and is designed for low NOx (nitrous oxides), CO, and unburnt hydrocarbon emissions. It is also highly desirable for this means to be simple to employ or add to already existing engines and to tune it for specific engines and installations.